The present invention relates to a radio receiver and an intermediate frequency signal generation method. For example, the present invention relates to a radio receiver converting a radio frequency signal to an intermediate frequency signal and receiving the intermediate frequency signal and an intermediate frequency signal generation method of such the radio receiver.
Japanese Unexamined Patent Application Publication No. 2007-158780 (Patent Literature 1) discloses a passband limiting device used in an FSK (Frequency Shift Keying) receiving apparatus. In the FSK receiving apparatus, a mixer mixes an RF (Radio Frequency) signal inputted through an antenna and a local oscillation signal, and generates an IF (Intermediate Frequency) signal. A BPF (Band Pass Filter) passes only the signal in a predetermined frequency band of the IF signal outputted from the mixer, and attenuates the signals in bands other than that, thereby band-limiting the IF signal. An AD (Analog to Digital) converter converts the band-limited IF signal to a digital signal.
The IF signal converted to the digital signal is further band-limited by using digital filters including an HPF (High Pass Filter) and an LPF (Low Pass Filter). Two types of filter coefficients can be set to each of the HPF and the LPF. A filter coefficient selection unit selects the filter coefficient of each of the HPF and the LPF, and switches the band width of a passband formed by using the HPF and the LPF between the band width of a wide band and the band width of a narrow band. The band width of the wide band is set in consideration of the temperature deviation, the change with time, and the like of a local oscillator assumed on the system. The band width of the narrow band is set in consideration of the signal spectrum of the IF signal and the like.
When an AFC (Automatic Frequency Control) operation is performed, the filter coefficient selection unit selects, from among the two types of filter coefficients that can be set to the HPF, the filter coefficient in which the high cut-off frequency is high, and selects, from among the two types of filter coefficients that can be set to the LPF, the filter coefficient in which the low cut-off frequency is low. In such a manner, the band width of the passband formed by using the HPF and the LPF is set to the wide band. After the AFC operation is performed, the filter coefficient selection unit selects, from among the two types of filter coefficients that can be set to the HPF, the filter coefficient in which the high cut-off frequency is low, and selects, from among the two types of filter coefficients that can be set to the LPF, the filter coefficient in which the low cut-off frequency is high. In such a manner, the bandwidth of the passband formed by using the HPF and the LPF is set to the narrow band.
In Patent Literature 1, in the AFC operation, the IF signal in the wide band is used to control the frequency of the local oscillation signal so that the frequency of the IF signal coincides with the center frequency of the HPF and the LPF. In Patent Literature 1, during the AFC operation, the passband width formed by using the HPF and the LPF is set to the wide band, so that even when the frequency of the IF signal is greatly shifted from the center frequency of the HPF and the LPF due to the temperature deviation, the secular deviation, and the like of the local oscillator, the AFC operation can be performed correctly. In addition, after the AFC operation, the passband width formed by using the HPF and the LPF is narrowed to coincide with the signal spectrum of the IF signal, so that even under the influence from the adjacent channel, the reception sensitivity can be prevented from being deteriorated.